Sven Oliver Grözinger

Quantification of interplay effects of scanned particle beams and moving targets.

Scanned particle beams and target motion interfere. This interplay leads to deterioration of the dose distribution. Experiments and a treatment planning study were performed to investigate interplay. Experiments were performed with moving radiographic films for different motion parameters. Resulting dose distributions were analyzed for homogeneity and dose coverage. The treatment planning study was based on the time-resolved computed tomography (4DCT) data of five lung tumor patients. Treatment plans with margins to account for respiratory motion were optimized, and resulting dose distributions for 108 different motion parameters for each patient were calculated. Data analysis for a single fraction was based on dose-volume histograms and the volume covered with 95% of the planned dose. Interplay deteriorated dose conformity and homogeneity (1-standard deviation/mean) in the experiments as well as in the treatment-planning study. The homogeneity on radiographic films was below approximately 80% for motion amplitudes of approximately 15 mm. For the treatment-planning study based on patient data, the target volume receiving at least 95% of the prescribed dose was on average (standard deviation) 71.0% (14.2%). Interplay of scanned particle beams and moving targets has severe impact on the resulting dose distributions. Fractionated treatment delivery potentially mitigates at least parts of these interplay effects. However, especially for small fraction numbers, e.g. hypo-fractionation, treatment of moving targets with scanned particle beams requires motion mitigation techniques such as rescanning, gating, or tracking.

Simulations to design an online motion compensation system for scanned particle beams

Respiration-induced target motion is a major problem in intensity-modulated radiation therapy. Beam segments are delivered serially to form the total dose distribution. In the presence of motion, the spatial relation between dose deposition from different segments will be lost. Usually, this results in over- and underdosage. Besides such interplay effects between target motion and dynamic beam delivery as known from photon therapy, changes in internal density have an impact on delivered dose for intensity-modulated charged particle therapy. In this study, we have analysed interplay effects between raster scanned carbon ion beams and target motion. Furthermore, the potential of an online motion strategy was assessed in several simulations. An extended version of the clinical treatment planning software was used to calculate dose distributions to moving targets with and without motion compensation. For motion compensation, each individual ion pencil beam tracked the planned target position in the lateral as well as longitudinal direction. Target translations and rotations, including changes in internal density, were simulated. Target motion simulating breathing resulted in severe degradation of delivered dose distributions. For example, for motion amplitudes of +/-15 mm, only 47% of the target volume received 80% of the planned dose. Unpredictability of resulting dose distributions was demonstrated by varying motion parameters. On the other hand, motion compensation allowed for dose distributions for moving targets comparable to those for static targets. Even limited compensation precision (standard deviation approximately 2 mm), introduced to simulate possible limitations of real-time target tracking, resulted in less than 3% loss in dose homogeneity.

Motion compensation with a scanned ion beam: a technical feasibility study

BackgroundIntrafractional motion results in local over- and under-dosage in particle therapy with a scanned beam. Scanned beam delivery offers the possibility to compensate target motion by tracking with the treatment beam.MethodsLateral motion components were compensated directly with the beam scanning system by adapting nominal beam positions according to the target motion. Longitudinal motion compensation to mitigate motion induced range changes was performed with a dedicated wedge system that adjusts effective particle energies at isocenter.ResultsLateral compensation performance was better than 1% for a homogeneous dose distribution when comparing irradiations of a stationary radiographic film and a moving film using motion compensation. The accuracy of longitudinal range compensation was well below 1 mm.ConclusionMotion compensation with scanned particle beams is technically feasible with high precision.

3D Online compensation of target motion with scanned particle beam

Intensity modulated, active beam delivery, like magnetic raster scanning, strongly relies on target immobilisation. If target motion cannot be avoided, interferences between the scanning procedure and the tumour displacement destroy the volume conformity. A prototype setup for 3D online motion compensation (3D-OMC) with a scanned particle beam is described, transferring the full potential of volume conformal irradiation to moving targets.